1. Field of the Invention
The present invention relates to a nonvolatile semiconductor memory device, and more particularly to an EEPROM capable of erasing electrical data.
2. Description of the Related Art
A conventional EEPROM is nonvolatile semiconductor memory device capable of erasing/writing electrical data.
Conventional EEPROMs include a tunnel region for electrically removing charges which represent storage information stored in a floating gate. In the latest EEPROM, a thin single-layer silicon oxide film is formed at the tunnel region. Charges, i.e., electrons, tunnel through the thin single-layer silicon oxide film and are transferred to a high potential region. For example, when a high voltage is applied to a drain region, electrons stored in a gate electrode tunnel through the single-layer silicon oxide film and are transferred to the drain region.
The most important reliability factor of conventional EEPROMS is an endurance characteristic of the tunnel insulating film i.e., a (repetition characteristic for erasing/writing data, which corresponds to repetitive operations for injecting electrons into a floating gate and removing the electrons from the floating gate).
The endurance characteristic depends on the quality of the tunnel insulating film. As described above, in conventional EEPROMs the single-layer silicon oxide film is used as the tunnel insulating film. At the present, the limit of the endurance characteristic is 10.sup.4 to 10.sup.5 repetitions in 64-Kbit EEPROMs.